Recently, communication technology has made considerable progress and there is a continuing trend to apply not only circuit switched communications but also packet switched communications. Also, terminals configured to operate according to one of those switching principles may get involved in a communication involving a terminal configured to operate according to another of those switching principles. In such cases, in order to maintain interoperability and compatibility, an interworking functionality has to be provided for.
Although the present invention is described subsequently with reference to circuit switched and packet switched communication, it is to be noted that these serve as examples only. Other switching principles may likewise be concerned by the present invention.
Still further, in order to describe specific embodiments of the present invention, reference is made to particular protocols currently already in use or under development. Any such reference to specific protocols and messages complying with such protocols are to be understood as an example only, and the present invention is not limited to those specific protocols/messages. Rather, any functionally similar protocol/message may be used in connection with the present invention.
Communication networks are generally divided in so-called domains. For the purpose of the present invention, a domain is distinguished from another domain based on the switching principle used in a respective domain, such as circuit switched or packet switched.
A transmission of data between a first and a second terminal is referred to as a session. A session comprises at least one transmission context, e.g. a PDP context (Packet Data Protocol) in case of a packet switched transmission or a similar context in case of a circuit switched transmission. Within a respective context, plural calls and/or channels may be present, e.g. plural packet flows in case of packet switched transmission or plural circuit switched calls in case of circuit switched transmission.
A particular focus in recent developments of communication networks (fixed networks such as the Internet as well as mobile networks such as a 3GPP network) is being laid on multimedia services. A multimedia service subscription enables a terminal to transmit data of different media types such as voice, video, or still image, sound, data “as such”, etc. Multimedia services often require real-time transmission in order to satisfy the needs of the end-users.
Both, i.e. circuit switched (CS) and packet switched IP (Internet Protocol) based (IMS) multimedia services are available in 3GPP networks (3rd generation Partnership Projects).
Several solutions have been specified for real-time circuit switched and packet switched video/multimedia communication such as telephony in fixed networks, as briefly outlined below.
Fixed Networks:
IP Based Fixed Networks:
Several approaches are available for a real-time multimedia transmission over IP networks, of which two approaches are briefly introduced below.
ITU-T has specified “Packet-based multimedia communications systems” in a recommendation.
In this case, the session is set up through the exchange of call control signaling messages over a TCP/IP (Transmission Control Protocol/Internet Protocol) connection, and the negotiation of capabilities is based on the use of the inband protocol control channel. The call control signaling is used for establishing a connection between two packet switched multimedia terminals. The inband protocol control signaling is used for capabilities exchange (i.e. codec negotiation), flow control, etc. between the terminals.
IETF (Internet Engineering task Force) has specified “Session Initiation Protocol (SIP)” which is an application-layer control/signaling protocol for creating, modifying and terminating (e.g.) multimedia sessions with one or more participants.
In the SIP/IETF case, the session is set up through the exchange of SIP messages, and the negotiation of capabilities is based on the exchange of session description protocol (SDP) elements over the SIP signaling.
A protocol stack on the user plane in the IP based multimedia communication is IP/UDP/RTP. Media streams (audio, video, data) are not multiplexed as in circuit switched CS multimedia, but are transported independently of each other over separate RTP sessions.
Circuit Switched Fixed Networks:
Several protocols have been developed by ITU-T for circuit switched (CS) video/multimedia services in PSTN (Public Switched Telecommunication Network) and ISDN (Integrated Services Digital Network).
The current situation is as follows:
CS video/multimedia solution: The basic CS video/multimedia solution is meant for a V.34 modem based video/multimedia communication over PSTN. The multiplexing of the media components is based on the framing protocol specified by ITU-T in a recommendation. The ITU inband protocol is used as the control protocol for the exchange of capabilities, e.g. for a codec negotiation.
extended CS video/multimedia solution: The extended CS video/multimedia solution is an extension to the basic CS video/multimedia solution and defines measures, such as the mandatory use of a retransmission protocol in the control channel and several robustness levels of the multiplexer, to be used in error-prone environments (like mobile networks). The measures are described in certain annexes of respective recommendations.
Another CS video/multimedia solution: Another CS video/multimedia solution is meant for video/multimedia communication over ISDN at data rates from 56 kbit/s to 1920 kbit/s. CS video/multimedia solution terminals use a specific user-network interface instead of the V.34 modem. Multiplexing, control channel and data transmission are as in the basic CS video/multimedia solution (see above). CS video/multimedia solution terminals support an inband negotiation mechanism to determine network end-to-end connectivity and to automatically negotiate a selected mode for the call among the CS video/multimedia solution modes, and voice telephone modes.
Still another CS video/multimedia solution: Still another CS video/multimedia solution is meant for video/multimedia communication over ISDN at data rates from 56 kbit/s to 1920 kbit/s. CS video/multimedia solution terminals use a specific user-network interface. The multiplexing of the media components is based on the framing protocol according to ITU-T recommendation. The inband protocol is used as the control protocol for the exchange of capabilities, e.g. for a codec negotiation.
The solutions support, either as mandatory or optional, at least some same codecs for audio and video. In some cases, i.e. when a given implementation does not support the optional codecs, transcoding is required.
Mobile Networks:
Conversational IP Based Mobile Networks:
3GPP specification release 5 (Rel-5) defines IP based multimedia services (IMS) that support real-time conversational multimedia over IP networks and a real-time packet switched streaming service (PSS) over IP networks.
The control plane of the conversational multimedia is based on 3GPP SIP, which is a 3GPP variant of the IETF SIP protocol. The session is set up through the exchange of SIP messages, and the negotiation of capabilities is based on the exchange of SDP elements over the SIP signaling. The protocol stack on the user plane is IP/UDP/RTP.
Streaming IP Based Mobile Networks:
3GPP specifications support both continuous streaming multimedia (media with an inherent notion of time, i.e. speech, audio, video, timed text) and discrete streaming multimedia (media that itself does not contain an element of time). For continuous media the protocol stack on the user plane is IP/UDP/RTP and the session control is based on RTSP and SDP. For discrete media the protocol stack on the user plane is IP/TCP/HTTP and the control is based on SMIL or SDP over HTTP, i.e. there is no separate session control protocol.
Circuit Switched Mobile Networks:
Circuit switched (CS) multimedia services in 3GPP networks, also known as CS video/multimedia solution, are based on the corresponding ITU-T recommendation, including an annex that defines the use of CS video/multimedia solution terminals in error-prone transmission environments like mobile networks. The transparent bearer service BS30, with a parameter indicating that the application is video/multimedia, is used in the mobile network. The multiplexing of the media components, based on the framing protocol ITU-T recommendation, is realized at the terminals. The inband protocol is used as the control protocol for the exchange of capabilities, e.g. for a codec negotiation. Additionally, higher end-to-end data rates can be achieved in 3G networks with the use of the multicall supplementary service.
Multicall in Mobile Networks:
The 3GPP Multicall supplementary service can be used for increasing the end-to-end data rate in multimedia calls in 3G networks (UTRAN). The core network handles the calls as independent calls. Both the network and the terminals (user equipments, UEs) need to support the Multicall supplementary service. The multiplexing of the multimedia signal to/from the connections is performed on the application level by the terminals. The Multicall supplementary service supports 2-6 simultaneous data calls for a terminal UE, e.g. 2 times 64 kbit/s=128 kbit/s, 6 times 64 kbit/s=384 kbit/s.
In case terminals operated according to such different switching principles are involved in a communication with each other, a certain interworking is required. In particular in connection with multimedia communications involving rather high transmission data rates, interworking may become complex.
Up to now, interworking between IP/CS multimedia communication has not been standardized in that there are no 3GPP standards, no IETF standards, and also no ITU-T standards.
In connection with video call conferencing, where a media gateway (MGW) and/or a conference unit (MCU, multipoint control unit) receives calls from several users participating in a conference call, each leg (some of which may be IP based and some ISDN multilink based) is terminated at the MGW/MCU and the transmission framing protocols and media coding (i.e. transcoding of media streams) is mapped. Further information may be found e.g. in corresponding ITU-T recommendations.
However, there is no end-to-end interworking at the session establishment.
Furthermore, video gateway solutions are currently offered by some manufacturers. Those solutions aim to support packet switched/circuit switched multimedia interworking. However, the documentation seen by the present inventor so far discusses only the mapping of basic protocols (packet switched multimedia, CS video/multimedia solution, transmission and codecs (i.e. transcoding). There are no hints of supporting interworking between domains of a communication network operated based on different switching principles.
In connection with multimedia calls, high data rates are not unusual. A high data rate means for example a data rate higher than 64 kbit/s. A video/multimedia call at such a high data rate, i.e. higher than 64 kbit/s (e.g. 128 kbit/s) uses—in a circuit switched domain or environment—the multicall supplementary service in circuit switched 3GPP mobile networks and a multilink operation in ISDN.
In both networks the “multi-operation” actually means separate calls for each max. 64 kbit/s portion of the total session, i.e. the calling entity establishes several calls to the same (called) B party. The sending entity (at each end) splits the data into the separate simultaneous channels/calls whereas the receiving entity combines the data received from the separate simultaneous calls (multicall, multilink).
Packet switched such as IP based multimedia transmissions typically uses one RTP session for each media component, i.e. one RTP session for audio, one for video, one for data, etc., no matter what the bandwidth requirement for each media component is.
Therefore, it is a problem how the interworking between domains of a communication network operated based on different switching principles such as CS/IP can be arranged, if the CS leg uses a multicall/multilink operation.